This invention relates to a method for coating metal with a dissimilar metal.
In general, it is well known that as a method for coating a parent metal such as aluminum with a dissimilar metal such as copper, there has been proposed such a method in which a pretreatment for removing an oxide film firmly produced on the surface of a parent metal is effected in an aqueous solution then substitution of heavy metal such as zinc, nickel, tin or the like for the oxide film is carried out to form a layer for coating its substrate; and electroplating or electroless plating is applied onto the resulting layer coated for the substrate. In such method, however, it is necessary to repeat etching steps by means of various acids or alkalis and rinsing steps many times, so that the operations become very complicated. Furthermore, the coated layer for the substrate obtained by this method has an inferior adherence to its parent metal, and there are many problems in corrosion resistance. Thus, in order to overcome such problems, methods for coating the surface of a parent metal with a dissimilar metal by utilizing a dry substituting reaction such as gassing, dipping, spreading, coating or the like process which will be described hereinbelow have heretofore been proposed.
As disclosed, for example, in Japanese Patent Publication No. 33253/74 "Process for the Production of a Composite Metallic Material of Aluminum or the Alloy Thereof", a conventional gassing process is characterized by preheating aluminum or an alloy thereof at a temperature of 350.degree. C.-600.degree. C., contacting the aluminum alloy thus preheated with a halide metal gas generated by heating an admixed halide metal together with a flux, reducing and removing the surface layer of the aluminum alloy, and at the same time diffusing and cementing a dissimilar metal in the halide metal gas on the aluminum alloy.
However, such a conventional gassing process has the following disadvantages (1)-(4). That is, (1) since a container for a fused salt which is utilized for gasifying a halide metal is made of metal or ceramics, there are many cases where a container made of ordinary metal cannot be used because of its remarkable corrosion. In the case of a ceramics container, it is difficult to manufacture a large container by the use of ceramics only. Accordingly, such a ceramics container must be constructed by means of ceramics blocks, but in this case a high protective technique against corrosion is required in the masonry joint. Furthermore, there is a large possibility that the masonry joints would be damaged by means of expansion and shrinkage of the halide metal in intermittent operation. (2) Jigs and other equipment or devices employed in the atmosphere gasified are remarkably attacked and wasted. (3) A rate of evaporation is very slow in a step for evaporating a halide metal by heating the same at the melting temperature thereof or more, so that it becomes a rate-determining factor for producing a coated layer and results in an unfavorable productivity rate. (4) It is necessary to provide an inert atmosphere such that the halide metal gas itself is not oxidized as the gas atmosphere.
As described, for instance, in Japanese Patent Laid-open No. 31164/80 "Process for the Production of Stainless Steel Coated with Copper", a conventional dipping process is characterized by dipping austenitic stainless steel in a molten bath of copper chloride containing an excess of copper and having a temperature of 450.degree. C.-700.degree. C. in a method for forming a copper coated layer on the surface of the austenitic stainless steel.
However, such a conventional dipping process involves the following disadvantages (1)-(2). Namely, (1) containers, jigs, and other equipment are readily corroded. (2) In the case when a metal coated with a dissimilar metal is withdrawn from a salt bath of halide metal after the completion of the reaction, the halide metal excessively adhered to a parent metal is carried away from the bath, so that loss of the halide metal increases. The halide metal thus brought out is discarded at the time of rinsing the parent metal so that it comes to nothing, and at the same time it requires much labor for such rinsing operation.
In the gassing and dipping processes as mentioned above, a halide metal is used in the gaseous or liquid form. Therefore, either process relates to the one in which a parent metal is directly contacted with the halide metal even without employing a binder to cause reaction in a hot condition, whereby the surface of the parent metal can be coated with a dissimilar metal.
However, since both of these processes have the above stated drawbacks, such a coating process by the use of a binder as described hereinbelow is practically utilized.
As disclosed, for instance, in Japanese Patent Publication No. 8161/68 "Process for Forming a Pinhole-free Alloy Layer on the Surface of a Parent Material of Aluminum or the Alloy Thereof", a conventional spreading process is characterized by comprising a first step of applying thinly an adherent material among hydrocarbons on the surface of an aluminum alloy of a parent material as a binder, a second step of spreading and permitting fine powders of a halide metal to adhere on the surface of the parent material after completing the first step, and a third step of heating the resulting product through the second step at a temperature at which aluminum halide sublimes in a furnace or a higher temperature and thereafter removing heat from the product.
However, such conventional spreading process involves also the following disadvantages (1)-(4). That is, (1) the process requires a binder for holding and permitting a halide metal salt to adhere to the surface of a parent metal. Further, a step for applying the binder to the surface of the parent metal is independently necessary for this process. Besides it is technically difficult to control the uniform application of the binder on only a necessary portion of the surface of the parent metal in proper quantities, in spite of the fact that the binder is in liquid- or paste-form. In addition, it is finally required to remove the binder from the product, resulting in increased loss in heating, and much labor is also necessary for the after-treatment. (2) The control of the amount of the halide metal spread and adhered onto the surface of the parent metal or the like operation is also technically very difficult in relation to the amount of the binder spread and the particle size of the halide metal. Especially, it is substantially impossible to control the spread or the like of the binder on the basis of the target thickness of the coated layer in its final product. Accordingly, an excessive binder is always spread in actual production. (3) The process requires heating of the parent metal under such condition that the binder and halide metal are permitted to adhere to the parent metal, so that the selection of heating method is restricted. Generally, the heating is effected by means of convection or radiation, but in such a heating method, it is very difficult to uniformly set a temperature rise in each portion of the surface of the parent metal to be treated. As a consequence, it results in dispersion at a time for starting reaction in each portion of the surface of the parent metal to be treated, so that there occurs scatter in the finish coated layer. (4) With the decrease in viscosity of the binder in the heating step, the degree of adhesion decreases so that the halide metal comes away and sags or runs, resulting in a nonuniform coated layer.
As disclosed, for example, in Japanese Patent Publication No. 23910/80 "Process for Coating the Surface of Aluminum or an Aluminum Alloy Parent Material with a Metal Layer", a conventional coating process is characterized by: uniformly coating the surface of the aluminum alloy parent material with a coating fluid prepared by adding a halide metal to be coated to a dispersion consisting of a hydrophobic solvent, aliphatic non-polar polymer and at least one trivalent alkyl amine; heating the aluminum alloy parent material thus coated at a temperature at which aluminum halide sublimes or a higher temperature to cause a substitution reaction between the halide of the metal to be coated and aluminum; and sublimating the aluminum halide thus produced, whereby a coated layer of a dissimilar metal is formed on the surface of the parent material.
However, such a conventional coating process involves the following drawbacks (1)-(4). Namely, (1) since the process requires previous preparation of a coating fluid consisting of a halide metal and binder, a particularly and highly skilled manner is necessary for kneading the halide metal with binder, and a homogeneously kneaded dispersing state cannot be obtained if an especially expensive dispersant is not used. Besides it is necessary that the properties of the coating fluid are always controlled in response to those required in a manner of coating in order to obtain a uniform coated layer. (2) Heating of the parent material coated with the coating fluid brings about nonuniformity in a distribution of temperature in case of, particularly, rapid heating, and it results in scatter in the quality of a product as in the case of disadvantage (3) of the aforesaid spreading process. (3) Even if a uniform coating was made on the surface of the parent material, sags and runs of the halide metal due to decrease of viscosity of the binder in the heating step cannot be avoided. Consequently, it is necessary to excessively coat the coating fluid by making allowance for a loss due to the sags and runs thereof. As a result, an amount of the halide metal on a treated surface of the parent material becomes finally nonuniform and scatter is generated in respect of the quality of the product. (4) Excessive energy and time are required for decomposing and removing the binder.